1. Field of the Invention
The present invention relates to the field of methods of maintaining planar layers in devices such as BAW (bulk acoustic wave) resonators, integrated capacitors and the like.
2. Prior Art
BAW (bulk acoustic wave) resonators are used to provide RF filters for wireless applications and reference oscillators. The acoustic mirror used in conjunction with a solidly mounted BAW resonator (SMR) consists of alternating layers having low and high acoustic impedance. The layer of high acoustic impedance, typically, but not limited to, a metal such as tungsten, is patterned over the layer of low acoustic impedance, typically, but not limited to, silicon dioxide. The topography resulting from the patterning is reduced through the application of another layer of the low acoustic impedance film which is then further processed to remove that portion of the film on top of the patterned high acoustic impedance layer, W. The thicknesses of the layers and placement of the mirror between the supporting substrate and the active BAW resonator is controlled such that energy from the desired acoustic waves generated in the resonator is reflected back into the device.
Prior art uses chemical mechanical polishing (CMP) (see U.S. Pat. No. 6,542,054) or an etch-back method (see U.S. Patent Application Publication No. US 2007/0266548) to reduce the topography associated with the patterning of the high acoustic impedance layer and application of the sacrificial upper layer of low acoustic impedance material. CMP processes are expensive and suffer from “dishing” defects that reduce the overall planarity of the final structure and may result in the propagation of undesirable acoustic modes in the device. The etch-back method struggles with the control of the size and shape of pits in the defect region formed at the edges of the patterned high acoustic impedance layer. These pits must be mitigated in subsequent process steps. The etch-back method also relies on the use of an etch-stop layer deposited beneath the high acoustic impedance layer to prevent etching of the low acoustic impedance material any deeper than the bottom plane of the high acoustic impedance layer.